The present invention relates to a binder for the dry hot coat foundry method essentially consisting of a solid resol type phenolic resin and to a process for producing resin-coated sand for use in the shell mold foundry process.
The resin-coated sand by dry hot coat method has conventionally been prepared by mixing heated sand with a novolac type phenolic resin (hereinafter referred to simply as novolac), then adding hexamethylenetetramine (hereinafter referred to as hexamine) as hardener in an amount of 10-15% by weight with respect to said resin in a state of aqueous solution under agitation, and further adding calcium stearate. Although such resin-coated sand prepared with novolac-hexamine is advantageous because of its fast hardening speed and fluidity, the hexamine employed in this method is decomposed by heat at the mold forming or metal casting processes to generate nitrogen compounds such as ammonia which eventually give rise to ecological pollution. Also, which, when such sand is utilized in the iron or steel casting, it gives rise to defects in the products such as pinholes or blowholes.
In order to reduce the above-mentioned drawbacks there have been made the following attempts to employ a binder completely free or almost free from nitrogen in the dry hot coat method.
(1) There has been proposed the use, as the hardening agent for the novolac, of a resol type phenolic resin, for example a liquid resol phenolic resin prepared with an alkali metal catalyst. This method, however, has a drawback of requiring a prolonged blending, and also the prepared resin-coated sand shows such defects as caking and peel-back at the mold forming. There has also been proposed the use, as another hardening agent, of a solid resol type phenolic resin prepared with ammonia catalyst (hereinafter referred to as solid ammonia resol), which however is associated with a serious drawback of a slow hardening speed:
(2) Some years ago there was proposed the use of a solid resol type phenolic resin (hereinafter referred to as solid resol) alone as the binder for use in the dry hot coat method. This method is disadvantageous in that the resin composition, though being provided with a slower hardening speed than in the case with novolac-hexamine, shows local hardening in the blending operation before the sand particles can be satisfactorily coated, thus resulting in an insufficient fluidity while hot. As a result, the shell mold prepared by resin-coated sand has an extremely low strength which could not be employed in practical use. This method, however, has been given consideration again because of the recent concern for the above-mentioned gas defects such as pinholes or blowholes and for ecological problems caused by the disagreeable odor at the mold forming or metal casting, both associated with the use of hexamine.
In order to improve the hardening speed, there have been proposals to control the reaction of solid ammonia resol and extract the solid ammonia resol in an earlier stage of reaction. Such solid ammonia resol, though realizable in a laboratory experiment, is difficult to obtain in uniform quality in a mass scale production due to the inevitable difficulty encountered in reaction control. Moreover the fluidity of sand while hot is still deficient with such solid ammonia resol, and the obtained mold therefrom is poor in strength. Also the solid ammonia resol prepared in this manner gives rise to another new drawback of caking of sand particles as the melting point of binder is inevitably lowered.
For the purpose of improving the hardening speed there has also been proposed the addition of a hardening accelerator such as resorcin, which however, constitutes a source of another disagreeable odor and still does not provide a fundamental improvement in the insufficient strength resulting from lack of sufficient fluidity while hot.